Electric blasting initiators



F. R. SEAVEY ErAL ELECTRIC BLASTING INITIATORS Filed Feb. 14, 1958 INVENTORS. ICK R. SEAVEY D. CLARK FREDER JOHN ATTORNEYS 0 June 28, 19 60 FIG 2,942,513 ELECTRIC BLASTING INITIATORS Frederick vR. Seavey, Alton, and John D. Clark, Herrin, Ill., assignors to Olin Mathieson Chemical Corporation, East Alton, 111., a corporation of Virginia Filed Feb. 14, 1958, Ser. No. 715,214

4 Claims. (Cl. 861) This invention relates to electric blasting initiators having bead igniters and to an improved method for their manufacture.

Electric explosive initiators generally consist of a cylindrical metal or plastic shell containing explosive or deflagrating material in association with an electrical firing device. The firing device is usually in'the form of a plug adapted to close the open mouth of the shell and a pair of lead wires passing through the plug and extending somewhat beyond its base and a high resistance bridge wire adjoining the lead wires and embedded in a thermal responsive ignition composition. When sufficient current is passed through the bridge wire, it becomes heated, thus actuating the ignition composition which, in turn, initiates the remaining charges in the shell.

Such initiators are manufactured by charging the explosive or deflagrating charge into the cylindrical metal or plastic shell which is closed at one end, and then sealing the open end of the charged shell with the bridge plug assembly. When these initiators are provided with a bead type igniter, the bead is formed by dipping the bridge wire into a paste-like slurry of the ignition composition in a solvent containing a dissolved binder. By this operation, the bridge wire and adjacent portions of the lead wires are completely coated by and embedded in theigm'tion composiiton. This is often accomplished in two or three steps, for example, whereby the sensitive igniter is deposited around the bridge wire, coated with a less sensitive, hotter burning booster layer and all covered by a protective and confining lacquer by a series of dips and at least partial drying between applications. After the head of ignition composition has been applied, the bridge plug assemblies are then dried. In order to assure proper functioning of the initiators having the bead-type igniter, it is common practice to put them in a special rack which is placed in a drying room, normally at a temperature of approximately 50 C. until the solvent is removed from the bead. This solvent removal often requires at least 8 hours, and in several. cases must be extended to about 24 hours. Thus, this involves a a serious interruption in the manufacturing process and also entails additional handlingof highly sensitive explosive materials. Therefore, the manufacture of electric initiators having bead-type igniters is normally slower and more hazardous than the manufacture of explosive initiators provided with loose igniters.

. It is, therefore, an object of this invention to provide a novel process for the manufacture of electric blasting initiators having bead type igniters overcoming the disadvantages of the prior art. Another object of this inventionis to effect economies and to reduce hazards in the manufacture of electric blasting initiators having bead igniters. A more specific object of this invention is to provide a process for the manufacture of electric explosive initiators inwhich the accumulation of sensitive explosive components is eliminated.

The manner in which these and other objects are accomplished by this invention will be fully understood when the following description is read in connection with the drawing, in which:

Figure 1 is a sectional view ofa detonataor bridge plug assembly illustrating one aspect of the invention; and

2,942,313 Patented June 28, 1960 Figure 2 is a sectional'view of an electric explosive initiator made in accordance with this inventon.

In accordance with this invention, generally stated, these and other objects are achieved by appling an igniter head to the bridge wire of a detonator firing circuit vand inserting the bridge Wire assembly carrying the solventladen head into a charged organic thermoplastic or thermosetting shell. Preferably, the bridge plug is also formed of an organic thermoplastic or thermosetting material. The ignition bead is formed by dipping the bridge wire into a paste-like slurry of the ignition com.-

position in an organic solvent containing a' dissolved binder. The plastic material forming the'detonator case and/or bridge plug must be capable of being permeated by the solvent in order to permit drying of the igniter bead, after assembly of the detonate 1 Thus, this invention provides a process by whichthe solvent-wet ignition bead can be inserted into a detonator assembly immediately after the ignition bead has been applied to the bridge wire, thereby avoiding the conventional drying period which has heretofore been deemed essential. After the detonator has been completely assembled, the solvent passes through the plastic detonator case and/or bridge plug, and a dry, firm, adherent igniter bead results. I a I The suspension or slurry of the ignition composition is prepared by mixing the composition with a lacquer for-med of one or more organic solvents and a binder. Any of the commonly usedignition compositions, such as the lead salts of nitroresorcinols, 'diazodinitrophenol i mercury fulminate, and the like,'can be used. Any sol uble organic binder compatible with the ignition com-- position can be utilized. The binder can be, for example, nitrocellulose, an organic plastic material identical to or compatible with the materials used in making the detonator case or plug, ethyl cellulose, polyvinyl ace tate, polymethyl styrene, ester gum, and the like. The solvents used in carrying out the process of the inven-- tion will vary with the particular binder used as well as with the material used in formulating the detonator case and/or plug. Various types of solvents, including ethyl acetate, acetone, tetrahydrofuran, acetonitrile, methyl ethyl ketone, secondary butyl alcohol, toluene, nitromethane, ethyl alcohol, and the like are suitable. In some cases, solvent mixtures are also applicable. For example, when the binder is nitrocellulose, an acetonealcohol mixture can be utilized; whereas if ethyl cellu- I ployed, it is common practice to adjust the viscosity of lose is used as a binder, a mixture of toluene, xylene, ethanol and butanol can be used. In' any event,'the solvent or solvent mixture must be substantially free of water. However, it is not essential that it be anhydrous and it is only necessary that it does not contain suflicient water to have a deleterious effect upon the detonator components.

In preparing the bead igniter, the suspension or slurry of the ignition composition has the consistency of a relatively thin paste. This consistency can vary widely and it is only necessary that it be sufficiently adherent to form a suitable head after the bridge wire had been dipped into it one or more times. The proportions between the ingredients of the slurry are also partially dependent upon the particular components used. In most instances, however, it has been found that the binder may constitute between about 1% .and 3% of the dry weight of the igniter composition. The binder is used in a suitable solvent and can be added as a dry powder to the igniter mixture which is then wet down withsolvent, or it can be dissolved or dispersed in a suitable amount of solvent before adding the other ingredients of the igniter mixture. When the-latter method is emthe mixture by either driving ofi solvent to raise the viscosity or by adding solvent to render the suspension more fluid.

The detonator case or shell, as well as the bridge plug closing the mouth of the shell, can be formed of any organic plastic materials permeable by the organic solvent present in the igniter bead. Suitable thermoplastic molding compositions include the cellulose esters such as cellulose acetate and cellulose acetate butyrate, cellulose ethers such as ethyl cellulose, polymerized vinyl compounds, polymerized esters of acrylic and substituted acrylic acids, polystyrene, chlorinated rubber, modified isomerized rubber, and the like. Thermosetting molding compositions based on resins of the phenol-formaldehyde, casein-formaldehyde, ureayformaldehyde, and phenol-furfural types are also readily adaptable for the formation of the bridge plug and detonator case. Molding compositions containing suitable plasticizers with or without fillers and coloring agents may be formulated from these and similar materials so that under proper molding conditions a shell and a bridge plug may be produced which have the desired physical properties.

The particular type of bridge plug assembly is not critical to the present invention. Thus, the ignition head can be formedon the bridge wire or it can be positioned about a pair of insulated metallic laminae to form the conventional match head type igniter. In addition, it has been found advantageous in some instancesto provide an ignition bead support adjacent the bridge wire by bending the lead wires in the neighborhood of the bridge wire or by providing the bridge plug with abasal extension. exemplified by US. Patent"2,.48'1,696, granted-September- 13,, 1949, to Frederick R. Seavey.

The manner in whichthe objects of the present invention are accomplished will be more readily understood by the following specific example of a preferred embodiment thereof with .referenceto the accompanying drawing;

A substantially non-aqueous slurry of normal lead styphnate inv a dispersion of ethyl cellulose was prepared by adding about 1 part by weight of ethyl cellulose to 99 parts by weight of normal lead styphnate, blending the two and, wetting the mixture with between 30 and 35wparts by weight of a mixed solvent containing 40 parts by volume toluene, 35 parts xylene, parts ethanol, and 10 parts butanol; After the ethyl cellulose was swollen and at least partially dispersed, intimate mixing was completed and the solvent content adjusted by adding the mixed solvent dropwise until a desirable creamy consistency was attained. As indicated above, if too fluid a mixture develops, its viscosity may be increased by drying during mixing. Molded ethyl cellulose bridge plugs 1 of the type shown in Figure 1- provided with lead wires 2 and 3 and ethyl cellulose insulation 4 on the lead wires extending into plug 1 were provided. Lead wire 2 was bent substantially at right angles to provide a lateral projection 5. The lead wires 2 and 3 were joined by a bridge wire 6. This assembly was dipped into the suspension of the ignition material so as to form an adherent bead'7 about the bridge wire and the adjacent end of the lead wires. Immediately after forming the ignitionbead on the bridge wire, the. bridge plugs were inserted into molded ethyl cellulose detonator cases 8 as shown in Figure 2. These cases were previously charged" with a compressed cyclonite base charge 9 and a lead azide' initiator 10. The plugs were sealed to the detonator cases by the application of a mixture of 80% toluene and secondary butyl alcohol to the annular space 11 between the top ofv the plug and the case. This solvent mixture flowed between the plug and case, bonding them together. The solvent blend migrated through. the shell wall and probably partially through. the bIldgELPlllg;Offllelassmblfida Structures of this type are particularly well A 1 detonators to leave firm, solvent-free, adherent igniter beads about the bridge wires. After the solvent had escaped, the firing characteristics of these detonators Were completely satisfactory; and no difference was observed between the functioning of detonators prepared in accordance with this invention and detonators prepared by the well-known method in which the igniter bead is thoroughly dried before assembly into the detonator.

While the detonator described in the above preferred embodiment was provided with a case, bridge plug, and bead ignition binder of ethyl cellulose, it will be understood that these components are not necessarily identical. Thus, the binder and bridge plug or binder and detonator case may be formed of the same material or, if so desired, three different and distinct materials can be used. Also, various types of explosive trains can be utilized in place of the cyclonite-azide-styphnate combination set forth in the above example.

The above description has been directed primarily to electric detonators. However, the invention is equally applicable to all types of electric blasting initiators, including instantaneous blasting caps, delay blasting caps, squibs, deflagrating or igniting devices, and the like.

Since variations in the specific embodiment which has been described may be within the spirit andscope of this invention, the'detailed description is not to be considered as limitative except in the light of the appended claims.

Having thus described the invention, what is claimed is:

l. A process for the manufacture of electric explosive initiators comprising providing an organic thermoplastic bridge plug having spaced lead wires extending therethrough and a bridge wire spanning the lead wires, charging an explosive train into an organic thermoplastic detonator case, preparing a suspension of'an ignition composition in a lacquer formed of a binder and a-s'olvent for the case, applying a bead of the suspension to the bridge wire, and assembling the thermoplastic case with the thermoplastic bridge plug carrying the bead before an' appreciable amount of solvent islostfrom the bead.

2. A process for the manufacture of electric explosive initiators comprising providing an organic thermoplastic bridge plug having spaced lead Wires extending therethrough and a bridge wire spanning the lead wires, charging an explosive train into an organic thermoplastic detonator case, preparing a suspension of an ignition composition in a lacquer formed of a binder and a solvent for the case, applying a bead of thesuspension to the bridge wire, and immediately sealing the bridge plug into the mouth of the plastic case.

3. A process for the manufacture of electricexplosive initiators comprising providing an organic plastic bridge plug having spaced lead wires extending therethrough and a bridge Wire spanning the lead wires, charging an explosive train into an organic plastic detonator case, preparing a suspension of an ignition composition in a solution of an organic binder, applying a bead ofthe suspension to the bridge Wire and immediately sealing the plastic case with the plastic bridge plug carrying the bead of the ignition composition, the bridge plug, detonator case and binder having substantially the same composition.

4. The process of claim 3 in which the bridge plug, detonator case and binder'for the ignition composition are formed of ethyl cellulose.

References'Cited in the file of this pat ent UNITED STATES PATENTS 2,399,034 Huyett et a1 Apr. 23, 1946 2,415,045 Seavey Jan. 28, 1947 2,848,950 Donaldson Aug. 26, 1958 FOREIGN PATENTS 528,918 Great Britain Nov. 11, 1940 

